As far as catalysts for copolymerization of ethylene with alpha-olefins are concerned, three properties of the catalysts are of a major importance:
1) molecular weight distributions of the resins produced with the catalysts, PA1 2) the response of the resin molecular weight to hydrogen, PA1 3) ability of the catalysts to effectively copolymerize ethylene and alpha-olefins.
One of the measures of the molecular weight distribution of an LLDPE resin is the melt flow ratio (MFR), which is the ratio of the high-load melt flow index HLMI or I.sub.21) to the melt index (I.sub.2) for a given resin: MFR=I.sub.21 /I.sub.2. The MFR value is believed to be an indication of the molecular weight distribution of a polymer: the higher the MFR value, the broader the molecular weight distribution. LLDPE resins usually have relatively low MFR values, e.g., of about 20 to about 45.
Molecular weight of ethylene copolymers can be controlled in a known manner, e.g., by using hydrogen. With the catalysts produced according to the present invention, molecular weight can be suitably controlled with hydrogen when the polymerization is carried out at temperatures from about 30.degree. to about 105.degree. C. This control may be evidenced by a measurable positive change in the melt flow indexes (I.sub.2 and I.sub.21) of the polymers produced. A relatively high sensitivity of the resin molecular weight to the amount of hydrogen present during a polymerization process is an important feature of the catalyst compositions of this invention.
Another important property of catalyst compositions for ethylene/alpha-olefin copolymerization is the ability thereof to effectively copolymerize ethylene with higher alpha-olefins, e.g., C.sub.3 -C.sub.10 alpha-olefins, to produce resins having low densities. This property of the catalyst composition is referred to as "higher alpha-olefin incorporation property" and is usually measured by determining the amount of a higher alpha-olefin (e.g., 1-butene, 1-hexene or 1-octene) required in a polymerization process to produce a copolymer of ethylene and the higher alpha-olefin having a given copolymer composition and a given density. The lesser is the amount of the higher alpha-olefin required to produce the resin of a given density, the higher are the production rates and, therefore, the lower is the cost of producing such a copolymer. Effective high alpha-olefin incorporation is especially important in the gas-phase fluid bed process, because relatively high concentrations of higher o-olefins in the fluid-bed reactor may cause poor particle fluidization.
It is an object of the present invention to provide a high-activity catalyst for copolymerization of ethylene and alpha-olefins yielding products of bimodal molecular weight distributions.
It is an additional object of the present invention to provide a catalytic process for copolymerizing ethylene and alpha-olefins which yields LLDPE of a bimodal molecular weight distribution at high productivity.
It is also an object of the present invention to provide a high activity catalyst compositions for the copolymerization of ethylene and alpha-olefins which exhibits relatively high melt flow index response to hydrogen.